1. Field of the Invention
The present invention relates generally to fuel fired furnaces and improvements in the operating efficiency of such furnaces and more particularly to improvements in radiant tube type fuel burner systems.
2. Description of the Prior Art
Radiant tube type fuel burner systems are well known in the art and are frequently employed, for example, in a heat treating furnace or annealing oven and in other processes where it is desired to segregate the combustion region from the region occupied by the product being treated. Frequently, the desire or goal is to avoid deleterious effects on the product by the combustion gases. A common way to accomplish this goal is to provide a so-called radiant tube having a burner unit near one end of the tube and an exhaust gas outlet at the other tube end with the tube being U-shaped or otherwise configured so as to pass into and out of a furnace. Thus, combustion flame propagates along the tube while being isolated from the furnace environment by the tube with this combustion heating the tube sidewalls and therefore also the furnace area.
It is also common practice to operate such radiant tube type fuel burner systems as negative pressure systems so as to avoid contaminating the product being treated in the furnace in the event that a leak occurs in the radiant tube. In other words, some scheme is employed to maintain the pressure within the radiant tube at or below the pressure within the product area of the furnace so that if the leak does occur product area atmosphere passes into the tube to be exhausted with the combustion products rather than combustion products leaking into the product area of the furnace. One scheme for achieving this negative pressure operation is to employ an eductor on the exhaust leg of the tube. Such an eductor is a Venturi-type device in the exhaust outlet receiving a stream of compressed air and thereby reducing the pressure in the exhaust system of the tube. With such an eductor, air is drawn into the firing leg of the tube due to this reduced pressure to be mixed with the gas supplied to the burner unit. When such a system is not burning, natural convection currents cause air to flow through the radiant tube undesirably cooling the tube and reducing overall furnace efficiency. Such a system also has radiation losses through both the firing and exhaust legs of the tube further reducing furnace efficiency.
It is also a common technique in a wide variety of combustion devices to attempt to harness the energy lost through the exhaust system when it expels high temperature exhaust gases. Since most combustion devices must heat the air of the incoming combustible fuel air mixture, a frequent technique is to employ a so-called recuperator which transfers heat from exhaust gases to incoming air to warm that air somewhat and reduce the fuel consumed in raising the intake air to its final temperature in the combustion process. A recent attempt to employ heat recuperative techniques in a radiant tube type fuel burner system is illustrated in U.S. Pat. No. 4,310,303. This patented arrangement suffers from several defects insofar as operating efficiency is concerned and also loses the exhaust gas isolating advantage of negative pressure systems by operating in a positive pressure mode where the input combustion air and input fuel are introduced into the tube at pressures exceeding the furnace environment pressure. Other prior patents include U.S. Pat. Nos. 2,841,383; 3,446,277; 3,814,174; 4,106,556; 4,222,824; 4,269,266 and 4,310,303; and Japanese Pat. No. 52-57551.